They do. sorta. It's definitely possible to put something like Starfield on a dual layer BDROM, probably even uncompressed! But then load times would be fucking crazy because BD is an order of magnitude slower than an SSD.
Distributing install files for a day 1 version of a game and using the disc as an auth key, (which is what they did last gen iirc) is still possible.
Transfer the BDROM to my SSD. Literally the same thing as downloading it online. I don’t need it to read off the disc while I play. 360 did this and it worked perfectly fine.
BDXL goes up to 128GB... Conveniently... According to everywhere I look... Starfield is 126.1GB for XBOX...
So yes... Discs do have the capacity and you're wrong.
Further, you can simply use compression, and unpack to the internal SSD. That can probably net you a bunch more space... and then you can move to 2-disc operations if your game is even larger than that.
I don't think many people do so I won't blame you for not knowing. I happen to know because I use something called m-disc for archival purposes. And those are just really fancy blu-rays at this point. The discs I use are 100GB and i knew there were bigger ones.